A gas discharge lamp, such as a fluorescent lamp, typically utilizes a power supply circuit to convert an a.c. line voltage to a high frequency bidirectional voltage which is impressed across a resonant load circuit containing the gas discharge lamp. The resonant load circuit includes a resonant inductor and a resonant capacitor for determining the frequency of resonance of current in the resonant load circuit. The power supply circuit includes a series half-bridge converter having a pair of switches that alternately connect one end of the resonant load circuit to a d.c. bus voltage and then to a ground, thereby impressing the mentioned bidirectional voltage across the resonant load circuit.
A prior art power supply circuit of the foregoing type is disclosed in co-pending U.S. patent application Ser. No. 08/020,275 attorney docket number LD 10,583), filed Feb. 18, 1993, entitled "Electronic Ballast Arrangement For A Compact Fluorescent Lamp," by Louis R. Nerone, the present inventor, and assigned to the present assignee and which is herein incorporated by reference. The disclosed power supply circuit utilizes feedback circuitry for controlling the mentioned pair of switches of the series half-bridge converter. The feedback circuitry operates in response to a feedback signal representing a current in the resonant load circuit.
By relying on feedback circuitry to control the switches, the power supply circuit of the foregoing patent application avoids the expense and bulk of extra circuitry for switch control. However, it would be desirable to reduce the level of variations in lamp power and lamp current that occur due to variations, for instance, in the line voltage.
A gas discharge lamp such as a low pressure fluorescent lamp, and the power supply or ballast circuit arrangement as it is more commonly known, are presently being offered on a wide scale commercial basis in a configuration that lends itself to being a viable energy efficient long life replacement for a conventional incandescent lamp. Compact fluorescent lamps as they are commonly known utilize a compact, typically multiple axis discharge vessel containing a gas fill which includes a mixture of mercury and a rare gas such as krypton or argon. The ballast circuit is contained in a housing base having an Edison Type screw base which can be installed in a conventional lamp socket. Because of the desirability of utilizing such compact fluorescent lamps as replacements for conventional incandescent lamps, it is necessary that the ballast circuit and the housing base occupy such a small space as would allow insertion in most light fixtures. To achieve this it is important that the size and quantities of the components that comprise the ballast circuit are kept to a minimum. For a discussion of the physical characteristics associated with disposing the ballast circuit within the housing base, reference is made to commonly assigned U.S. patent application Ser. No. 07/766,608 filed on Feb. 26, 1991 by Minarczyk et al. which is herein incorporated by reference.
In addition to the desirability of utilizing this improved power supply circuit for the popular compact fluorescent lamps which have an electroded arrangement for exciting the discharge, it would be advantageous if this circuit arrangement could be utilized on an electrodeless fluorescent lamp where the discharge is excited by introduction of an RF signal which is coupled to the medium through an excitation coil disposed in close proximity to the medium.